This invention relates to supervisory systems and more particularly to a receiver of a protective relaying modem. Such a system would typically be used with monitors and circuit breakers in a power transmission system to detect and isolate a fault in a particular power transmission line system. Such protective systems have been found advantageous for detecting phase or ground faults in power lines especially when such power lines are located in and span very remote and non-accessible areas.
The detection and subsequent isolation of a faulty power line requires the transmission and reception over some communication medium, of a so-called "trip" signal indicating that a fault has occurred. Under normal or non-fault operating conditions, a second or "guard" signal is transmitted and received as indicative of such operation. Typically the trip and guard signals are the same in amplitude but predeterminably vary in frequency within a designated frequency channel. The equipment which codes the "trip" signal for transmission over the communication channel and decodes the received signal to a "trip" output alarm is called a protective relaying modem.
At the receiving location, the conventional technique has been to pass the received, composite signal through a narrow bandpass L-C filter to reject frequency channels not under consideration, and then utilizing frequency selective circuits such as discriminators, recover the modulation signals. The L-C type filter, in particular, has the disadvantage of being very bulky in size in addition to requiring, for overall frequency stability in the receiver, temperature compensation which is very complex and thus expensive to attain.
An additional problem faced by these prior receivers is that they are always subject to bursts of electronic noise which could generate a pseudo trip condition. The existence of this detrimental noise is dependent upon the particular location and environment in which the receiver is used and the internal characteristics of the receiver itself. It is also known that a receiver's susceptibility to noise is inversely proportional to the amount of time allocated to detect a fault signal before a fault condition is indicated and acted upon. Keeping in mind that it is desired to detect a fault as soon as it occurs, it is clear that for certain environments a shorter detection time can be utilized to render dependable information, while in other so-called noisy environments a longer detection time is required for dependability of detection. The ability of the receiver to efficiently adapt to various noise generating environments has not heretofore been available.
It is, therefore, desirable to provide a communication receiver which dependably detects a trip signal (indicating for example a phase or ground fault on a power transmission line), is adaptable for efficient use in various noise generating environments and which is susceptible of being placed into operation with as few critically controlling parameters as possible.